Understanding Reference Ranges and Units on a Laboratory Report

Section Editor: Christopher McCudden Ph.D., DABCC, FADLM, FCACB
September 22, 2026

Every laboratory report shows more than your result. Beside each measurement is a reference range, and after the number is a unit. Together, they turn a bare figure into something you can interpret. On its own, a creatinine of 1.0 or a sodium of 139 tells you nothing at all.

This article explains what a reference range is, where it comes from, and why it is not the same thing as a healthy range. It also explains the units used on laboratory reports, and why the same result can be printed as two very different numbers. The last sections cover the flags beside your result and the things other than disease that can change a number.

The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal (or unexpectedly normal) result with your doctor.

What is a reference range?

A reference range is the span of results expected in a healthy population. Laboratories determine it by testing a group of people screened as healthy, then taking the middle 95 percent of their results. The lowest 2.5 percent and the highest 2.5 percent of that healthy group fall outside the range. The international guideline most laboratories follow, CLSI EP28-A3c, asks for at least 120 healthy people in each group.

You may also see a reference range called a reference interval or a normal range. Most laboratories now prefer the term reference range. The word normal suggests that everyone inside the range is healthy and everyone outside it is not, and neither is true.

Why a result outside the reference range does not always mean something is wrong

Because a reference range is built from the middle 95 percent of healthy people, about 1 healthy person in 20 falls outside it on any given test. That is not a laboratory mistake. It is a direct consequence of how the range is defined.

This matters most when many tests are reported together. A basic metabolic panel reports eight measurements, and a complete blood count reports more. If ten measurements are reported at once, there is roughly a 40 percent chance that at least one will be flagged in a person with nothing wrong. With twenty measurements, the chance is closer to two in three.

How far outside the range a result sits matters more than whether it is outside at all. A value a fraction above the upper limit carries much less weight than one well above it. Repeating the test often brings a borderline result back inside the range, because every measurement varies slightly from one day to the next and from one run to the next.

What can change a result apart from disease?

A result can fall outside the reference range for reasons unrelated to illness. Knowing which applies often explains a flagged number without further testing.

Whether you fasted, glucose and triglycerides in particular are strongly affected by what you ate or drank in the hours before the draw.
The time of day. Some measurements follow a daily rhythm so that the same person can be within range in the morning and outside it in the afternoon.
Recent hard exercise. Muscle enzymes can stay elevated for a few days after heavy exertion, which can look like liver or heart injury on a report.
How much you had to drink. Dehydration concentrates the blood, which can nudge several measurements up without anything changing in the body.
Medications and supplements. High-dose biotin, sold as a hair and nail supplement, interferes with many laboratory tests and can push a result in either direction. Tell the laboratory or your doctor what you take.
Problems with the sample itself. A difficult blood draw can damage red blood cells and release their contents into the sample, which falsely raises potassium. Samples that clot or sit too long before testing can be affected in other ways.

If one of these explains your result, your doctor will usually repeat the test under better conditions rather than investigate further.

Why reference ranges differ from one laboratory to another

Two laboratories can test the same sample and print different reference ranges beside the result, and both can be correct. The range depends on the equipment and the chemical method used, and these differ between laboratories. It also depends on the population each laboratory serves.

Ranges also differ by age and by sex, and some change during pregnancy. Ranges for children often differ from adult ranges, and for some tests they shift as a child grows. For all of these reasons, comparing your result with a range you found online, or with the range on an older report from a different laboratory, can be misleading.

What the units on your report mean

A result only means something with its unit attached, and laboratories use two different systems. In the United States, most laboratories report in conventional units, such as milligrams per deciliter (mg/dL). In Canada, the United Kingdom, and most of Europe, laboratories report in SI units, such as micromoles per liter (µmol/L).

The same result can look completely different in the two systems. A creatinine of 1.0 mg/dL is the same as 88 µmol/L. A glucose of 100 mg/dL is the same as 5.6 mmol/L. A hemoglobin of 14 g/dL is the same as 140 g/L. Nothing about the sample has changed, only the scale the result is reported on. This matters if you are comparing your result with information written for another country.

What the flags and symbols beside your result mean

Most laboratory reports flag any result that falls outside the reference range. The letter H means the result is above the upper limit, and L means it is below the lower limit. Some laboratories use A for abnormal, an asterisk, or bold or different-colored text.

A second kind of flag marks a critical result, sometimes shown as HH or LL or as the word critical. These are values far enough outside the range to need attention the same day. The laboratory telephones them to the ordering doctor rather than waiting for the report to be delivered. If you can see a critical flag on a report in a patient portal, your doctor has usually already been called.

When your result is compared with a target instead of a reference range

Not every number on a laboratory report is read against a reference range. For some tests, the comparison is with a treatment target or a diagnostic cutoff, which is a different kind of number.

Cholesterol. LDL cholesterol on a lipid panel is judged against a target chosen for your cardiovascular risk, so two people with the same result can be given different advice.
Hemoglobin A1c. The HbA1c result is compared with diagnostic cutoffs used to identify prediabetes and diabetes, and with a personal target if you already have diabetes.
INR. On a coagulation panel, the INR target depends on why you are taking an anticoagulant, and your doctor sets it rather than the laboratory.
Medication levels. These are compared with the range in which the drug is expected to work without causing harm.

On tests like these, a result inside the printed range is not automatically the goal, and a result outside it is not automatically a problem.

What a result inside the reference range does and does not tell you

A result inside the reference range is reassuring, but it does not rule out disease. Some conditions produce normal results in their early stages. A result can also be normal for the population and still be unusual for you, which is why your own previous results are the most useful comparison your doctor has.

The reverse is true as well. A result that has moved steadily within the range, from near the bottom to near the top over a year, can matter even though it was never flagged. This is one reason your doctor may repeat a test that looked normal.

Questions to ask your doctor

What is the reference range for this test at the laboratory that ran it?
How far outside the range is my result?
Could my age, my sex, or a pregnancy change the range that applies to me?
Is this result being compared with a reference range or with a treatment target?
How does this result compare with my previous results?
Could anything about how the sample was collected have affected the result?
Does this test need to be repeated, and if so, when?
Which results on this report, if any, need action now?
Should I be doing anything differently because of this result?
What would make you want to see me sooner than we planned?